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Huahua's Tech Road

花花酱 LeetCode 263. Ugly Number

题目大意:如果一个数的素数因子只有2,3,5,我们称它为丑数。让你判断一个数是不是丑数。

Problem:

Write a program to check whether a given number is an ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5. For example, 6, 8 are ugly while 14 is not ugly since it includes another prime factor 7.

Note that 1 is typically treated as an ugly number.

 

Idea:

Math

Solution:

C++

Related Problems:

花花酱 167. Two Sum II – Input array is sorted

题目大意:给你一个排过序的数组,让你输出两个数的index,他们的和等于target。

Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution and you may not use the same element twice.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

Solution:

C++ / two pointers

Time complexity: O(n)

Space complexity: O(1)

 

C++ / Binary search

Related Problems:

 

花花酱 LeetCode 755. Pour Water

题目大意:

给你地形的高度,有V单位的水会从K位置落下,问你稳定之后水位的高度。

Problem:

We are given an elevation map, heights[i] representing the height of the terrain at that index. The width at each index is 1. After V units of water fall at index K, how much water is at each index?

Water first drops at index K and rests on top of the highest terrain or water at that index. Then, it flows according to the following rules:

 

  • If the droplet would eventually fall by moving left, then move left.
  • Otherwise, if the droplet would eventually fall by moving right, then move right.
  • Otherwise, rise at it’s current position.

Here, “eventually fall” means that the droplet will eventually be at a lower level if it moves in that direction. Also, “level” means the height of the terrain plus any water in that column.

 

We can assume there’s infinitely high terrain on the two sides out of bounds of the array. Also, there could not be partial water being spread out evenly on more than 1 grid block – each unit of water has to be in exactly one block.

Idea:



Solution 1: Simulation

Time complexity: O(V*n)

Space complexity: O(1)

C++

Java

Python3

花花酱 LeetCode 309. Best Time to Buy and Sell Stock with Cooldown

题目大意:给你每天的股价,没有交易次数限制,但是卖出后要休息一天才能再买进。问你最大收益是多少?

Say you have an array for which the ith element is the price of a given stock on day i.

Design an algorithm to find the maximum profit. You may complete as many transactions as you like (ie, buy one and sell one share of the stock multiple times) with the following restrictions:

  • You may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).
  • After you sell your stock, you cannot buy stock on next day. (ie, cooldown 1 day)

Example:

Idea:

DP

Solution:

Time complexity: O(n)

Space complexity: O(1)

C++

Related Problems:

花花酱 Fenwick Tree / Binary Indexed Tree SP3

本期节目中我们介绍了Fenwick Tree/Binary Indexed Tree/树状数组的原理和实现以及它在leetcode中的应用。
In this episode, we will introduce Fenwick Tree/Binary Indexed Tree, its idea and implementation and show its applications in leetcode.

Fenwick Tree is mainly designed for solving the single point update range sum problems. e.g. what’s the sum between i-th and j-th element while the values of the elements are mutable.

Init the tree (include building all prefix sums) takes O(nlogn)

Update the value of an element takes O(logn)

Query the range sum takes O(logn)

Space complexity: O(n)


Applications:

Implementation:

C++

Java

Python3